Ambulance bosses have drawn up plans to deal with an ‘inevitable’ flu pandemic that could affect a third of all people – including children and usually fit young adults.

They lay bare the crippling impact an outbreak would have, and reveal how it would be impossible to stop the virus spreading rapidly – even if it starts outside the UK.

Half of all paramedics in the north west would likely contract the flu, with schools and nurseries advised to shut to reduce the spread of infection amongst pupils.

999 operators would be swamped by calls, including by the ‘worried well’, while antiviral drugs could run out.

The disaster would last for up to five months - with the virus also likely to re-emerge.

However, the North West Ambulance Service (NWAS) has come up with contingency plans designed to help the NHS cope in the event of a deadly outbreak.

A spokesman said NHS England requires all ambulance services to have a plan in place, and said it would help bosses ‘best manage the demand, supplement the workforce in the event of high levels of sickness, and work with other healthcare partners to offer the best possible care to patients’.

Influenza pandemics have happened at irregular intervals throughout history, including three in the last century, the report said.

The outbreak in 1918, dubbed the Spanish flu, claimed 20 million lives worldwide, including 150,000 people in the UK.

Pandemics are unpredictable and can happen at any time of the year – not just in the winter. They happen when a new strain of the virus emerges, to which most people are susceptible.

Experts believe it would spread rapidly in the UK over the span of two to three weeks, before a second wave of illness six to nine months later.

In 2008, the body of Sir Mark Sykes, a diplomat, politician and landowner was exhumed by scientists who hoped it could help prevent another pandemic.

Sir Sykes died of Spanish Flu in France in 1919, aged 39. He was buried in a lead-lined coffin and virologists thought flu particles would remain intact as a result – giving them a valuable insight into the flu strain. However, the coffin had split and the cadaver badly decomposed.